Prokaryotic cells regulate gene expression so they can make needed proteins efficiently and respond to changing conditions. This lesson introduces operons and explains how the trp operon uses a repressor to control tryptophan production.
Prokaryotic DNA is generally organized as a circular chromosome that is supercoiled in the cell’s nucleoid region. Genes that contribute to the same function or biochemical pathway are often grouped together in an operon. This arrangement allows related genes to be controlled together.
All flashcards
What is an operon?
A group of related genes and associated regulatory DNA sequences that can be regulated together; its genes may be transcribed into one mRNA.
How does tryptophan act as a corepressor?
It binds the trp repressor, changing its shape so the repressor can bind the operator and inhibit transcription.
What happens to the trp operon when tryptophan is scarce?
The repressor cannot bind the operator, so RNA polymerase can transcribe the genes for tryptophan-biosynthesis enzymes.
How does allolactose affect the lac repressor?
Allolactose binds the repressor and changes its shape so it can no longer bind the operator.
How does cAMP–CAP promote transcription?
The complex binds near the promoter and helps RNA polymerase bind or remain there, increasing transcription.
Quiz questions
Why is the trp operon classified as repressible?Answer: Tryptophan activates its repressor, which can then bind the operator and switch transcription off.
What does allolactose do in lac operon regulation?Answer: It binds the lac repressor, preventing the repressor from binding the operator.
Under which conditions is lac operon transcription expected to be high?Answer: Lactose is present and glucose is absent.
What is the function of the lacZ gene product?Answer: Beta-galactosidase breaks lactose down into glucose and galactose.
What is the role of the cAMP–CAP complex in alternative-sugar operons?Answer: It binds near the promoter and helps RNA polymerase initiate transcription.
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.